Enveloped viruses use specialized proteins present at the virus surface to translocate their genetic material across the host cell membrane during infection. For coronaviruses, homotrimers of the spike glycoprotein promote host cell attachment and fusion of the viral and host membranes. Although coronaviruses have a significant pandemic potential, the lack of high-resolution data for any coronavirus spike trimer limits our mechanistic understanding of infection by this family of viruses. The objective of the proposed work is to obtain high-resolution snapshots corresponding to the various stages of the fusion reaction mediated by coronavirus spikes and to study the structural determinants associated with antibody inhibition of viral infection. In Aim I, we propose to elucidate the architecture of the Mouse Hepatitis Virus (MHV) pre-fusion spike using cryoEM. Aim II will be dedicated to studying the conformational changes associated with the fusion reaction with an emphasis on the first intermediate (extended intermediate) and the post-fusion spike. In Aim III, we will characterize the 3D organization of human coronavirus spikes to understand how these viruses overcome the species barrier and to identify structurally conserved regions that could be potential targets for therapeutic initiatives. The final aim will rely on structure-guided protein design to engineer antibodies targeting human coronavirus spikes with the goal of identifying immunogens for raising broadly-neutralizing antibodies.